The overall goal of the proposed research is to understand how the neural system coordinates axial trunk movement. We are currently unable to directly determine the forces produced by muscle activation, nor accurately measure morphology and motion of the skeletal system in-vivo. However, using both inverse and forward-dynamics modeling techniques allows investigation of these variables. We may then explore the relationship between the mechanical requirements of the task, and the coordination necessary to perform the task. The first specific aim is to identify the critical factors that determine success or failure of a normal sit-up. Using the model for simulation allows us to determine the sensitivity of the movement outcome to changes in motor control strategy such as altered muscle activation timing and/or intensity. The second specific aim is to characterize task failure in selected groups of subjects. Approximately 5% of the population have difficulties performing tasks requiring movement of the axial trunk, such as the situp. This is possibly due to a mechanical disadvantage caused by their morphology, or by inappropriate coordination of the task. Farther simulation will enable us to determine the changes in motor strategy required to complete the task. In addition, normal subjects have been shown to be able to adapt their motor strategy to overcome novel mechanical conditions. By modeling the changes that occurred, we may begin to understand how the neural system organizes the motor control strategy to perform axial movement.